what is ASA?
Make sure you have connection to the windows servers, and open firewall rules to allow connection for check_nt to talk to NSClient++ service on windows boxes. Default port for NSClient++ is 12489.

two options.
1) setup a site-to-site vpn so you can communicate with the hosts
or
2) setup a second nagios server at the remote site that will work with your current nagios server. setup a nat for that server so that your current server can communicate with the remote one.

this doesn't replace the fact that you need the NSClient++ that kosarajudeepak mentioned to monitor windows servers in depth (e.g. beyond pinging or checking if a tcp port is open and listening; it is used to query for perf mon stats and such)

Yes you can change the port# Step 7 in the link i sent you will talk about it
7. Edit the NSC.INI file (located in the C:\NSClient++ directory) and make the following changes:
Uncomment all the modules listed in the [modules] section, except for CheckWMI.dll and RemoteConfiguration.dll
Optionally require a password for clients by changing the 'password' option in the [Settings] section.
Uncomment the 'allowed_hosts' option in the [Settings] section. Add the IP address of the Nagios server to this line, or leave it blank to allow all hosts to connect.
Make sure the 'port' option in the [NSClient] section is uncommented and set to '12489' (the default port).

S check_nt has different option you pass
check_nt -H Hostname -p Port
For example if u have connections comes to firewall with 123 port mapping to server1 then u do the checks on server as
check_nt -H -p 1234
Similarly
check_nt -H -p 5678
etc....

I never tried this kind of single host public ip scenario, but logistics should work
Check the following for more options.

# ./check_nt --helpcheck_nt v1991 (nagios-plugins 1.4.12)Copyright (c) 2000 Yves Rubin (rubiyz@yahoo.com)Copyright (c) 2000-2007 Nagios Plugin Development Team <nagiosplug-devel@lists.sourceforge.net>This plugin collects data from the NSClient service running on aWindows NT/2000/XP/2003 server.Usage:check_nt -H host -v variable [-p port] [-w warning] [-c critical][-l params] [-d SHOWALL] [-t timeout]Options: -h, --help Print detailed help screen -V, --version Print version informationOptions: -H, --hostname=HOST Name of the host to check -p, --port=INTEGER Optional port number (default: 1248) -s <password> Password needed for the request -w, --warning=INTEGER Threshold which will result in a warning status -c, --critical=INTEGER Threshold which will result in a critical status -t, --timeout=INTEGER Seconds before connection attempt times out (default: 10) -h, --help Print this help screen -V, --version Print version information -v, --variable=STRING Variable to checkValid variables are: CLIENTVERSION = Get the NSClient version If -l <version> is specified, will return warning if versions differ. CPULOAD = Average CPU load on last x minutes. Request a -l parameter with the following syntax: -l <minutes range>,<warning threshold>,<critical threshold>. <minute range> should be less than 24*60. Thresholds are percentage and up to 10 requests can be done in one shot. ie: -l 60,90,95,120,90,95 UPTIME = Get the uptime of the machine. No specific parameters. No warning or critical threshold USEDDISKSPACE = Size and percentage of disk use. Request a -l parameter containing the drive letter only. Warning and critical thresholds can be specified with -w and -c. MEMUSE = Memory use. Warning and critical thresholds can be specified with -w and -c. SERVICESTATE = Check the state of one or several services. Request a -l parameters with the following syntax: -l <service1>,<service2>,<service3>,... You can specify -d SHOWALL in case you want to see working services in the returned string. PROCSTATE = Check if one or several process are running. Same syntax as SERVICESTATE. COUNTER = Check any performance counter of Windows NT/2000. Request a -l parameters with the following syntax: -l "\\<performance object>\\counter","<description> The <description> parameter is optional and is given to a printf output command which requires a float parameter. If <description> does not include "%%", it is used as a label. Some examples: "Paging file usage is %%.2f %%%%" "%%.f %%%% paging file used." INSTANCES = Check any performance counter object of Windows NT/2000. Syntax: check_nt -H <hostname> -p <port> -v INSTANCES -l <counter object> <counter object> is a Windows Perfmon Counter object (eg. Process), if it is two words, it should be enclosed in quotes The returned results will be a comma-separated list of instances on the selected computer for that object. The purpose of this is to be run from command line to determine what instances are available for monitoring without having to log onto the Windows server to run Perfmon directly. It can also be used in scripts that automatically create Nagios service configuration files. Some examples: check_nt -H 192.168.1.1 -p 1248 -v INSTANCES -l ProcessNotes: - The NSClient service should be running on the server to get any information (http://nsclient.ready2run.nl). - Critical thresholds should be lower than warning thresholds - Default port 1248 is sometimes in use by other services. The error output when this happens contains "Cannot map xxxxx to protocol number". One fix for this is to change the port to something else on check_nt and on the client service it's connecting to.Send email to nagios-users@lists.sourceforge.net if you have questionsregarding use of this software. To submit patches or suggest improvements,send email to nagiosplug-devel@lists.sourceforge.net

I too am looking into this. However, because the port used to communicate is defined on both the server nagios config and widnows client, i see no way to do this other to use a plugin like NRPE. Now that isn;t to say that you cannot add a variable to each of the check_nt command configs ala this one for SSH:

add -p xxxx for whatever port you'd like to use to monitor the windows box. make the same port change in the nsc.ini file to match the port you set in windows.cfg.
add a simple port forward to firewall to wanted server via rotuer/asa/whatever

Just make sure to incriment ports accordingly. 12489 is used by default, so I started with 124850

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